The repeating subunits that make up DNA are called nucleotides. A nucleotide is an organic molecule that helps provide the structure for both DNA and RNA strands.
Nucleotides are comprised of a nitrogenous base, a phosphate group and a five-carbon sugar that is either ribose or deoxyribose. Without a phosphate, the subunit is a nucleoside. In DNA structures, nucleotides use the sugar deoxyribose. The bases for DNA structures are adenine, thymine, cytosine or guanine. In RNA structures, nucleotides use the sugar ribose and uracil instead of thymine. In addition to forming DNA and RNA strands, nucleotides are a key part of metabolism, as they carry energy within the cells.